Superconducting magnets are capable of operating in a persistent state, where no power is dissipated due to zero electrical resistance to the electrical current flowing through the magnet coils. In order to ramp the current flowing through the magnet coils up to the desired amperage so as to produce a magnetic field of the desired strength, the coils are connected to a power supply through power leads which dissipate energy and prevent the persistent mode of magnet operation. After ramping, the magnet terminals are shorted out with a superconducting switch to complete the circuit for the current flowing through the magnet coils to achieve the persistent state.
This method of ramping up superconducting magnets is well known as are superconducting switches for providing the superconducting link between the magnet terminals for persistent state operation after the magnet has been ramped up. Such a superconducting switch consists of superconductors which are warmed to their normal (non-superconducting) state during ramp-up operation and are then cooled to the superconducting state for persistent mode operation. Energy is dissipated in the switch from quench heaters used to drive the switch normal prior to ramp-up and then from a voltage imposed by the power supply across the switch while in the normal state during ramp-up. Depending on the type of switch conductors, the dissipated energy can be quite substantial, which results in local relatively high temperatures.
In cryogenless conduction cooled magnets, the heat dissipated by the switch presents a particular problem. The heat from the switch diffuses to the main coil support structure because it is a large cold mass. In cryogenless type magnets, the large cooling capacity of liquid helium is not available. Refrigeration is provided by a cryocooler with limited cooling power. Therefore, so that the cryocooler is not overburdened in cooling the switch in proportion to the remainder of the magnet, it is desirable to thermally isolate the switch from the main coil support structure.
However, when the magnet attains its operating current during ramping, the switch needs to be cooled down to its superconducting state in order for the magnet to be persistent. In cryogenless type magnets, it is most desirable to cool the switch using the cold stage of the cryocooler, which is at odds with efficiently using the cooling capacity of the cryocooler for the main coils, so as to prevent the main coils from going from the superconductive state to the normal state, also known as a quench.